The human leg is a complex mechanism, absorbing and dissipating the impact forces generated by supporting and moving the body. There are high impact axial loads with acceleration and deceleration even in activities of daily living. For instance when standing one half body weight goes through each knee. While walking two and one half body weight goes through each knee with each step as the person slightly sways side to side. Getting out of a chair without help of the arms increases the axial forces across the knee almost twice body weight. Damage may occur with work or activities of daily living.
High impact sports, such as running and tennis are known to significantly increase loads on weight-bearing joints. As such, sports injuries commonly involve damage to the knees, ankles and hips. Even sports previously considered low impact can generate significant loads on weight-bearing joints. For example, golf is considered by many to be low impact sport; however, the golf swing at ball impact typically generates loads of about 3.5 to 4.5 times the golfer's weight on the knees. Interestingly these loads are simultaneously transmitted to both knees at impact. These increased loads are generated from the impact of the club having a long lever arm hitting a ball while the player's muscles are contracting to secure footing or fixation to the ground. Thus, even those that actively participate in sports such as golf are susceptible to injury of weight-bearing joints.
Even minor imbalances in the foot that are not harmful or even detectable under usual circumstances can make one more vulnerable to injury. Imbalances may result in the body compensating or overcompensating in an attempt to equalize balance. Such compensation or overcompensation may result in fatigue, which is known to increase risk of injury. In addition proper imbalance may reduce the efficiency of muscle development and may decrease the body's mechanical efficiency when participating in sports or other activities.
Risk of damage to the body is not limited to those that participate in sports. A variety of adverse knee, ankle, foot and hip medical conditions are prevalent among the general population and in particular among the aging population. In fact, as the world's population ages, these conditions will become more widespread—while only 1 out of every 20 people was age 65 or older in 1950, by 2050 that number will increase to 1 out of 6.
Scientists have recently established a link between a protein that declines with age and the development of osteoarthritis (OA), a common disease of aging affecting nearly 27 million Americans1. Specifically, the loss of the protein (HMGB2; found in the surface layer of joint cartilage) leads to the progressive deterioration of the cartilage—the hallmark of OA. Cartilage is the tissue layer that sustains joint loading (weight bearing) and allows motion at joint surfaces. Whereas normal cartilage provides a durable, low-friction, load-bearing surface, damaged cartilage significantly reduces mobility. Currently, no effective treatment for this degenerative disease exists, apart from palliative drugs for pain and inflammation.
OA typically begins with a disruption of the surface layer of cartilage, called the superficial zone. Functionally, of the four layers of cartilage present in joints, this is the most important. In non-diseased joints the cartilage surface is smooth, enabling joint surfaces to interact without friction. However, the cartilage of the superficial zone begins to deteriorate as OA progresses triggering an irreversible process that eventually leads to the loss of underlying layers of cartilage. The fragments of cartilage are dispersed in the joint causing reaction of the joint lining, inflammation and the symptoms of pain and swelling. Over time, bone surfaces begin to grind painfully against one another.
The knee is the most common lower limb site for OA, with the disease affecting the tibiofemoral and patellofemoral joints either in isolation or combination, with the medial tibiofemoral compartment as the most commonly affected. Patients with knee OA report knee pain and difficulty with walking, stair-climbing and housekeeping3.
Management strategies for knee OA can be regarded as primary (reducing risk factors to lessen disease incidence); secondary (intervening to slow or prevent progression to serious disease); or tertiary (treating pain and disability)4. To date, most knee OA research has focused on tertiary strategies relating to pain management. Among these strategies, the primary emphasis has been on drug therapies, which typically include unwanted side effects and can be costly5.
Currently, no cure exists for OA, and joint replacement is the only established treatment for end-stage OA. In the case of the knee, the cost for such an operation is high and estimated $35,000 for those without health insurance. The operation also typically entails a 3-7 day hospital stay. During the surgery the doctor assesses the condition of the joint surfaces, removing damaged bone and cartilage, and implanting new joint surfaces made of plastic and metal. These new joint surfaces are not permanent, and will likely need to be replaced after 10 to 15 years. Thus, slowing the disease's progression is essential to reducing its impact both personally and upon society, as a slower disease progression rate would, for many patients, eliminate the need for the joint replacement procedure entirely.
As if OA itself were not troubling enough, a recent study published in BioMed Central's open access journal, Arthritis Research & Therapy, found that increased waist circumference and body mass index (BMI) were associated with the risk of both knee and hip joint replacement. Further, in addition to the increased joint loading caused by the excess baggage accumulating around the world's waistlines, the adipose tissue itself can release cytokines that have been implicated in joint damage6. Cytokines can act to accelerate progression of OA by contributing to the deterioration of cartilage and hastening the onset of bone/bone contact.
Gel and cushioned insoles as well as heel wedges and unloader braces have been proposed to decrease knee, ankle and foot pain by unloading forces on the joint. However, most insoles act merely to alleviate pain while doing little to treat the injury or to prevent progression of OA. Specifically, they are a component of tertiary management strategies designed to manage pain. However, studies find that while insoles may provide some cushion or softening, they often do not provide continual support. For instance, cushion insoles tend to bottom out or lose their contour when under load or increased load. As such, the cushion may provide some comfort but may not reduce peak axial load on the joint. In fact the gel type insoles may actually increase the peak axial load because the foot at impact rapidly compresses the material on way to impact rather than modulating or absorbing the person's weight. Further, while heel wedges have been proposed to unload the joint, not all experts support the use of these insoles to help patients suffering from arthritis. For example, in the case of symptomatic medial compartmental OA of the knee, the official stance of the American Academy of Orthopedic Surgeons (AAOS) is to refrain from prescribing lateral heel wedges, as their systematic review of the wedges provided no “evidence that lateral heel wedges are more effective than neutral heel wedges, when assessed with the WOMAC instrument for up to 24 months.” The AAOS' “Full Guideline” for treatment of osteoarthritis of the knee (Dec. 6, 2008) went on to state “[t]hese data suggest that there is no benefit to using lateral heel wedges, and there is the possibility that those who do not use them may experience fewer OA of the knee symptoms.” Thus, conventional insoles and heel wedges, including lateral heel wedges have not been deemed effective as an OA treatment. In addition, this report goes on by stating while unloader braces have also been proposed there is no clear evidence in the literature of their effectiveness.
While developing primary management strategies for OA could be difficult, especially given its link to aging and decreasing levels of certain molecules, secondary strategies, including those designed to slow the progression of the disease, could be extremely helpful. Further, with increased interest in sports and increased lifespan, there exist a need to develop new noninvasive devices and methods for the prevention and treatment of injuries and medical conditions related to weight-bearing joints, including the knee, ankle, foot and hip.